Clinical and laboratory studies were conducted to determine the etiological agents and neurological manifestations of acquired immunodeficiency syndromes in man (AIDS) and in nonhuman primates (SAIDS). Patients with neurological complications of AIDS have been admitted to the Neurology Service of the NIH Clinical Center for study. Patients admitted to other Institutes have been seen by the Infectious Diseases Branch Consultation Service. Patients have been evaluated to determine the spectrum of neurological illenesses found in AIDS. Appropriate virological and immunological studies are being conducted by IDB and collaborating laboratories. Findings from studies of nonhuman primates with SAIDS are being compared with those obtained through our AIDS protocols. Clinical and immunological parameters of SAIDS are being evaluated. Transmission of SAIDS to normal, uninfected rhesus monkeys using filtered serum and plasma, tissue homogenates or filtered plasma or serum from monkeys with SAIDS has been successfully completed. A type D retrovirus related to Mason Pfizer monkey virus has been found to be the etiologic agent of SAIDS.